Question

If x varies directly as 2y and x=7 when y=1 find x when y=3

Answer 1

Given that x varies directly as 2y  that is

x α 2y      [ means x is directly proportional to 2y ]

⇒ x= k(2y)        [ where k is a constant ]

⇒ x = 2ky       ...(1)

when x = 7 and y = 1

so 7 = 2k

⇒ k = 7/2

Substitute the value of k in (1), we get

So x = 2(7/2)y

⇒  x = 7y

Now when y = 3 then

x = 7(3) = 21